<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <script src="https://cdn.bootcss.com/jquery/3.3.1/jquery.js"></script>
    <style>
    input{
        width: 200px;
    }
    </style>
</head>
<body>
    <input type="text" id="txt">
    <script>
        $("#txt").focus(function(){
            $(this).animate({width:"300px"},1000)
        })
        $("#txt").blur(function(){
            $(this).animate({width:"200px"},1000)
        })
    </script>
</body>
</html>